A trophic level is a feeding position in a food chain, and a pyramid of biomass shows the total dry mass of organisms at each level. Biomass always decreases from one level to the next because most energy is lost through respiration, heat, waste, and uneaten material rather than being passed on.
What is a trophic level?
A trophic level is a step in a food chain representing a group of organisms that obtain their energy from the same source. The word trophic comes from the Greek for feeding.
There are typically four trophic levels in most food chains:
| Trophic level | Name | Examples | Energy source |
|---|---|---|---|
| Level 1 | Producers | Grass, oak trees, algae | Sunlight (photosynthesis) |
| Level 2 | Primary consumers | Rabbits, caterpillars, zooplankton | Eating producers |
| Level 3 | Secondary consumers | Foxes, thrushes, small fish | Eating primary consumers |
| Level 4 | Tertiary consumers | Owls, large predatory fish, eagles | Eating secondary consumers |
Producers are always plants, algae, or other photosynthetic organisms — they capture the Sun's energy and convert it into biological molecules. All other organisms are consumers.
What is a pyramid of biomass?
A pyramid of biomass is a diagram that shows the total dry mass of all organisms at each trophic level in an ecosystem. The width of each bar represents the mass at that level; the producer bar is always at the bottom.
Biomass is measured as dry mass (the mass remaining after all water has been removed) in units of grams per square metre (g/m²) or kilograms per square metre (kg/m²). Dry mass is used because water content varies enormously between organisms and does not represent the actual biological material.
A typical pyramid of biomass is widest at the base and narrows toward the top — a genuine pyramid shape — because biomass is lost at each level.
Why does biomass decrease at each trophic level?
When an organism eats another, not all the biomass of the prey becomes biomass of the predator. Most energy is lost in the following ways:
- Respiration — energy released as heat to maintain body temperature and fuel life processes; this energy cannot be passed on.
- Excretion and egestion — waste products (urine, faeces) contain organic molecules that are not absorbed.
- Uneaten material — bones, shells, roots, and feathers may never be consumed.
- Movement — energy is used in locomotion, which generates heat.
As a rough rule of thumb, only about 10% of the biomass at one trophic level is transferred to the next. If a meadow has 10,000 g/m² of grass (level 1), there might be around 1,000 g/m² of grasshoppers (level 2), 100 g/m² of shrews (level 3), and only 10 g/m² of kestrels (level 4).
How do you draw a pyramid of biomass?
To draw a pyramid of biomass from data:
- Identify the organisms at each trophic level from the food chain.
- Record the biomass value (dry mass per unit area) for each level.
- Draw horizontal bars to scale, centred on a vertical axis — the producer bar is widest and at the bottom.
- Label each bar with the organism name and trophic level.
- Add a title and scale bar.
Worked example:
| Level | Organism | Biomass (g/m²) | Bar width (scale: 1 cm per 500 g/m²) |
|---|---|---|---|
| 1 — Producer | Grass | 5,000 | 10 cm |
| 2 — Primary consumer | Voles | 500 | 1 cm |
| 3 — Secondary consumer | Barn owls | 50 | 0.1 cm |
The dramatic narrowing from level 1 to level 3 illustrates how little energy reaches the top of the food chain.
How does a pyramid of biomass differ from a pyramid of numbers?
A pyramid of numbers counts the total number of individual organisms at each level. This can produce unusual shapes: a single oak tree supports thousands of caterpillars, so the producer bar (1 tree) is narrower than the primary consumer bar (many caterpillars) — an inverted pyramid that does not represent energy flow accurately.
A pyramid of biomass avoids this problem because it uses mass rather than number. One oak tree has far greater biomass than one caterpillar, so the producer bar is always the widest, giving a true pyramid shape that reflects energy content. Pyramids of biomass are therefore a more reliable way to compare ecosystem energy at different levels.
Why do food chains rarely have more than five trophic levels?
By the fifth trophic level, so little biomass remains — roughly 0.001% of the original producer biomass — that there is not enough energy to support a population of top predators. Each link in the chain loses approximately 90% of the energy, so very long chains are energetically impossible. This is why ecosystems with fewer trophic levels can support much larger total populations.
Frequently asked questions
What is the difference between a trophic level and a food chain?
A food chain shows the specific feeding relationships between named organisms (e.g., grass → rabbit → fox). A trophic level is the position (level 1, 2, 3…) occupied in that sequence. Multiple species can share the same trophic level — rabbits and deer are both primary consumers, for example — so a food web (multiple interconnected food chains) captures ecosystem relationships more completely than a single chain.
Why is biomass measured as dry mass?
Organisms vary enormously in their water content — a jellyfish is more than 95% water, while a seed may be less than 10%. If wet mass were used, the pyramid would reflect water content rather than biological material. Dry mass (after removing all water) represents the actual organic matter available as food and energy for the next trophic level, making it a consistent and meaningful measure.
Why is only about 10% of biomass transferred between trophic levels?
Most of the energy taken in at one trophic level is used for the organism's own respiration, lost as heat, excreted as waste, or remains in parts of the organism that are never eaten. Only the biomass that is actually eaten and digested — and successfully incorporated into the consumer's own body tissues — passes to the next level. On average, this amounts to roughly 10% of the total biomass at the lower level, though the exact percentage varies by ecosystem.
Can a pyramid of biomass ever be inverted?
In most ecosystems, pyramids of biomass are true pyramid shapes (widest at the base). However, in certain aquatic ecosystems — particularly open ocean — phytoplankton (microscopic producers) reproduce very rapidly but exist at low total biomass at any one instant, while zooplankton consumers can accumulate more biomass. This can produce a temporarily inverted biomass pyramid. It does not mean more energy is available to consumers; it simply reflects the rapid turnover rate of producers rather than their total production.
For Socratic KS3 biology with Professor Darwin — tracing energy from leaf to top predator across scales of organism, ecosystem, and planet — visit aitutors.me.